Possible bug? Sculpt Mode -> Simplify Brush is affecting far side of mesh
It can be a bug, or something in developement.

For you question: You can use various Mask tools / operations to temporally "block" the areas from being affected. For example, There is a brush type called Mask (or press M to switch to it), and also a menu category called Hide/Mask on the 3d View header when in Sculpt mode.

However, to quickly meet what you need, you can ShiftCtrlLMB to drag the block area (it will be shown in dark grey). Then just paint. When finishing, clear the mask by AltM.